The present invention relates to methods and apparatus for removing cyclic noise caused by vibration of a cryogenic refrigerator when a magnetic sensor is cooled by the cryogenic refrigerator, the magnetic sensor being a superconductive quantum interference device (hereinafter referred to as SQUID) magnetic sensor including a SQUID which is in a superconductive state under a cryogenic temperature level, and the like.
In the past, a SQUID utilizing the Josephson effect has been a known superconductive device. When magnetic flux input circuitry including a superconductive pick up coil is interconnected to the SQUID, a SQUID gradiometer (a magnetic sensor based on a SQUID) is obtained, which can measure extremely weak magnetic field such as magnetic field caused by small currents flowing in a living organism, magnetic field caused by micro magnetic substances in a living organism, and the like.
When a SQUID gradiometer is to be cooled to a cryogenic temperature level, that is a temperature level for transiting a SQUID and a superconductive coil to superconductive state, a method is known which stores liquid helium at the cryogenic temperature level in a helium liquefied temperature insulated casing (cryostat), then dips the SQUID gradiometer in the liquid helium. When this method is employed, a refrigerating device of a cryogenic refrigerator for generating cold temperature is usually housed in the helium liquefied temperature insulated casing so as to condense and liquefy the helium gas which is vaporised in the helium liquefied temperature insulated casing.
When this method is employed, a SQUID gradiometer is cooled rapidly and stable for the entire extent of the SQUID gradiometer because the SQUID gradiometer is dipped in liquid helium. But, disadvantages arise in that the cooling system for cooling a SQUID gradiometer becomes large in size, and that operability of the cooling system is lowered because helium is housed in the helium liquefied temperature insulated casing for cooling a SQUID gradiometer. By taking these disadvantages into consideration, a new method has received attention, which cools a SQUID gradiometer by contacting the SQUID gradiometer with a refrigerating device of a cryogenic refrigerator in a direct heat transferrable manner (refer to Japanese Patent Laid Open Hei 2-302680).
When this new method is employed, the cryogenic refrigerator includes some sections which vibrate when the cryogenic refrigerator is driven, and it is very difficult to remove the vibration entirely. A disadvantage arises in that the noise caused by the vibration of the cryogenic refrigerator contaminates the output signal of a SQUID gradiometer so as to make the output signal inaccurate in original signal detection of the SQUID gradiometer.
The disadvantage can be dissolved by employing two channels in the signal system of the SQUID gradiometer, one channel being used for usual measurement, the other channel being used for measuring the noise caused by the vibration of a cryogenic refrigerator, and by comparing both measurement signals. A new disadvantage arises in that the number of channels of the signal system increases.
This disadvantage can also be dissolved by employing one channel of the signal system of a SQUID gradiometer and by taking the ususal measurements, and then measuring the noise caused by the vibration of the cryogenic refrigerator, and thereafter comparing both measurement signals. A new disadvantage arises in that accurate detection of the original signal is difficult because the measurement timings are quite different from one another, causing inevitably varying vibration characteristics of the cryogenic refrigerator.
When a magnetic sensor other than a SQUID gradiometer is employed, similar disadvantages to those above-mentioned arise when the magnetic sensor measures flux under a cooled condition from a cryogenic refrigerator.